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Simple Radio is a communications program for DCS World that ties into the in-game radios and allows realistic multiplayer voice comms using the frequencies and capabilities of the radios in whichever aircraft you are currently using.
It works with multiple radios so, for example, in a large multiplayer flight you and your wingmen can use one radio frequency for yourselves for intraflight communication, and tune another to a frequency that all players are connected to so you can effectively communicate across a large number of players without stepping on one another.
There are a couple of prerequisites you will need to install before installing Simple Radio (referred to as SRS from here on)